PROJECT SUMMARY A traumatic knee injury in adolescence and early adulthood results in an abrupt halt to sports activity. The subsequent trajectories of a return to sports and their effects on long-term physical activity behavior, weight gain, and ultimately knee osteoarthritis (OA) are not known. One likely consequence is weight gain. Obesity, in addition to knee injury, is a known risk factor for the development and progression of knee OA. Thus, weight gain following injury may accelerate the development of OA and increase the risk for early joint replacement and functional decline. The long-term objective of this fellowship application is to prepare the applicant to be a successful independent scientist focused on the consequences of injury in early adulthood in order to prevent/delay the development of chronic disease and optimize long-term quality of life. As a step towards this goal, the objective of this proposal is to evaluate sports activity, physical activity, weight gain, and the development of knee OA after knee injury. The central hypothesis is that reductions in sports activity after knee injury contribute to weight gain, which subsequently increases the risk of inactivity and incident knee OA in the long-term. This proposal capitalizes on an on-going longitudinal study of clinical and functional outcomes in individuals after knee injury, specifically anterior cruciate injury and reconstruction (n=150). Outcome data were collected at baseline (soon after injury), and at the 6-month, 1-year, 2-year, 5-year follow-ups. 10-year follow-up data collection is on-going. Aim 1 will determine the trajectories of sports activity from 1 to 5 years after knee injury and the association of these trajectories with physical activity at 10-years. Aim 2 will investigate changes in body weight immediately (baseline to 6 months) and in the short-term (1 to 5 years) after knee injury, and how weight change relates to sports activity. Aim 3 will estimate the association of sports activity and weight status with incident knee OA after knee injury. The results of this proposal will provide a critical understanding of potential health consequences related to inactivity after injury, and serve as impetus to develop a long-term strategy to maintain knee health after injury through physical activity and healthy weight, which is not the focus of current post-injury intervention. By completing this fellowship under the guidance of experienced mentors in a strong environment, the applicant will be prepared to launch a career as an independent scientist investigating consequences of musculoskeletal injury.